The Two-Way

4:23 pm

Fri April 25, 2014

For New York, The '10-Year Storm' Isn't What It Used To Be

Sandbags protect the front of the New York Stock Exchange on Oct. 29, 2012, in preparation for Hurricane Sandy.

Richard DrewAP

New York City is 20 times more likely to flood during a storm than it was in the mid-1800s, partly owing to sea-level rise linked to global climate change, according to a new study.

The maximum water height at New York Harbor during storms such as Hurricane Sandy has risen nearly 2.5 feet since 1844, says the study, which was published in a recent issue of Geophysical Research Letters.

"Specifically, [lead author Stefan] Talke explained, there's a 10 percent chance today that, in any given year, a storm tide in New York harbor will reach a maximum height of nearly two meters (about six and a half feet), the so-called '10-year storm.' In the mid-19th century, however, that maximum height was about 1.7 meters (about 5.6 feet), or nearly a foot lower than it is today, according to tide gauge data going back to 1844, he noted.

" 'What we are finding is that the 10-year storm tide of your great-, great-grandparents is not the same as the 10-year storm tide of today,' Talke said."

"Three of the nine highest recorded water levels in the New York Harbor (NYH) region have occurred since 2010 (Mar. 2010, Aug. 2011, and Oct. 2012), and eight of the largest twenty have occurred since 1990," the study says.

But, as National Geographic points out, "The cause ... is not just global sea-level rise: That accounts for less than a foot and a half (around 0.4 meters) of the increase."

"Global climate change and local changes in the harbor itself are two possible causes of the increased height of storm surges in New York, Talke says. 'If I had to wager a guess, I would say it is probably a combination, but that remains to be seen,' he says."

"The rise in sea level and storm tide combined puts the odds of storm waters overtopping Manhattan's defenses at one in every 4 to 5 years, compared to only once in every 100 to 400 years in the 19th century, the study found. (Put another way, the annual chance of a storm overtopping the seawall has gone from about 1 percent to 20 to 25 percent.)

"That high storm tide — more than 4 feet higher than the previous record set in December 1992 and the largest since 1821 — was created by a 9.4-foot storm surge and the evening high tide during a full moon, when tides are higher than normal (though the evening high tide was not as large as the morning one)."

"Knowing that there has been an increase in storm tides and figuring out why the increase occurred could help scientists better predict what will happen in the coming decades and help cities mitigate future problems," Talke says.